Chronostasis

after the Romans, historical records argue against large migrations following the Roman occupation or the Norman conquest, and the prevailing view is that these represent elite dominance. Historical and archaeological records suggest some Viking settlement in the 9th century in various parts of Britain but does not support a substantial displacement of existing people. Currently, one of the most hotly debated transitions is that from Roman Britain to Anglo-Saxon Britain. This transition was once widely accepted as providing clear evidence of a mass migration but this has recently been questioned and thought possibly, once again, due to a small number of elite Germanic immigrants. Previous work on genetic data has been interpreted to suggest historic migrations but new work by Michael Weale, Deborah Weiss and colleagues at University College London, the University of California at Davis and the Free University of Amsterdam in Molecular Biology and Evolution (19, 1008–1021 (2002)) exploits new genetic markers to take a fresh look at the issues. They studied the non-recombining portion of the Y chromosome and the mitochondrial genome because of their potential to provide high-resolution haplotypes. The team believed the Y chromosome is particularly useful because it has many slowly mutating biallelic markers to help resolve long-standing genealogical relationships as well as rapidly mutating microsatellite markers to help study more recent events. Mouth swabs were obtained from 313 males in 7 British towns that were approximately 50 km apart along an east–west transect through central England and north Wales. The towns are all longestablished market towns mentioned in the Domesday book of 1086 which the researchers thought would have been subject to less recent migration than large cities. Samples were only taken from men if they and their paternal grandfather had been born within 30 km of these towns. Samples were also taken from males in Norway and Friesland because of their different role in Britain’s immigration history. Friesland is thought to be a site of Anglo-Saxon immigration because of its position and because its language is considered closest to Old English. Norway is one source of Viking invaders. From their results, the researchers defined seven haplogroups on the basis of unique event polymorphisms (UEPs), which together with the microsatellite markers defined 150 haplotypes. The study then analyzed the genetic data under a range of flexible population genetic models by which it was possible to test both mass migration scenarios and lower levels of continuous gene flow to best explain the data. The results suggest that the striking patterns they obtained are best explained by a substantial migration of Anglo-Saxon Y chromosomes into central England but not into Wales. This would explain both the similarity of haplotypes in males from central England and Friesland and the striking difference between them and the Welsh samples. Anglo-Saxon settlements and culture appeared throughout England, but importantly, did not extend into north Wales, where many of the Celtic Britons living in England are thought to have fled. Conflict between the Welsh and Anglo-Saxon kingdoms continued over a long period and resulted in Offa’s Dyke, a 240 km long ditch barrier constructed in AD 740 as a boundary between England and Wales. The linguistic, cultural, and political separation of the two continued for the next 500 years until King Edward I’s defeat of Llywelyn II in 1282. The genetic data suggest a restricted malemediated gene flow between the two regions over the past approximately 1500 years. Remarkably, the genetic differentiation appears still discernable in the present day. A mass migration and its limits may have been a key feature of Britain’s first millennium. Quick Guide